Compression apparatus with variable speed screw and method

Abstract

An apparatus is provided for compressing a loose solid feedstock. The apparatus includes a two stage compressor. The first compression stage is a screw compressor. The second compressor stage is a reciprocating compressor. The reciprocating compressor operates co-axially with, and receives its feed from, the screw compressor. A choke cone maintains pressure in the outfeed from the compressor stages. The reciprocating compressor, the screw compressor, and the choke cone are all adjustable in real time to control the compression of the feedstock according to a pre-programmed schedule that need not have equal compression and retraction strokes. The operation of the screw compressor may be advanced or eased off depending on the motion of the reciprocating compressor. The operation of the choke cone may be actively controlled to obtain a coordination with the compressor stages.

I claim: 
     
       1. A two stage compressor feed apparatus operable to compress loose feedstock material, said feed apparatus comprising:
 a first compressor stage and a second compressor stage; 
 said first compressor stage having a screw, said screw having a volute operable to drive the feedstock forward in an axial direction while compressing the feedstock; 
 said second compressor stage having an axial accommodation permitting an end of said screw to extend therethrough; 
 said first compressor stage having a drive connected to operate said screw; 
 said drive being a variable speed drive; 
 sensors mounted to monitor operation of said screw and said second compressor stage; 
 a controller operable to receive signals from said sensors and to control operation of said screw; and 
 said controller being operable continuously to vary operating speed of said drive in response to operation of said second compressor stage: 
 said second compressor stage has a duty cycle, said duty cycle including a first portion and a second portion; 
 during said first portion of said second compressor stage duty cycle said controller is programmed to urge said screw to operate in a first speed regime, and in said second portion of said second compressor stage duty cycle said controller is programmed to operate said screw in a second, different speed regime; 
 said first portion of said dut cycle is a piston advancing portion, and said second stage of said duty cycle is a piston retracting portion; and 
 said controller is programmed to reduce speed of said screw during said piston retracting portion of said duty cycle, and to increase speed of said screw during said piston advancing portion of said duty cycle. 
 
     
     
       2. The two stage compressor feed apparatus of  claim 1  wherein said screw has a volute, and pitch between flights of said volute varies therealong. 
     
     
       3. The two stage compressor feed apparatus of  claim 1  wherein said pitch varies continuously along said screw, said pitch decreasing in spacing toward a distal tip of said screw. 
     
     
       4. The two stage compression apparatus of  claim 1  wherein said second compressor stage includes a piston, said piston being axially reciprocable relative to said screw. 
     
     
       5. The two stage compression apparatus of  claim 1  wherein:
 said first stage screw has a discharge tip, said discharge tip being surrounded by a sleeve; 
 said sleeve being an axially stationary sleeve; 
 said second stage piston surrounding said sleeve, and being axially reciprocable relative thereto; 
 said sleeve having an interior face oriented toward said screw; and 
 said interior face of said sleeve having axially extending reliefs defined therein. 
 
     
     
       6. The two stage compression apparatus of  claim 1  wherein said controller is programmed to monitor electrical motor current to said drive, and to maintain said electrical motor current below a maximum value during operation of said apparatus. 
     
     
       7. The two stage compression apparatus of  claim 1  wherein:
 said screw has a proximal end mounted to said drive and a distal tip distant therefrom; 
 said second compressor stage is a piston compressor stage having a piston mounted co-axially with said screw; 
 a stationary sleeve is mounted co-axially about said distal tip of said screw, said sleeve having an internally axially grooved wall facing said screw; and 
 said piston is an annular piston mounted about said sleeve for axial reciprocation relative thereto. 
 
     
     
       8. The two stage compression apparatus of  claim 7  wherein said apparatus includes a de-watering zone, and said screw discharges through said piston into said de-watering zone. 
     
     
       9. A two stage compression process for compressing loose feedstock material, comprising:
 providing a first compressor stage and a second compressor stage, said first compressor stage having a screw, said screw having a volute operable to drive the feedstock forward in an axial direction while compressing the feedstock; 
 said second compressor stage having an axial accommodation permitting an end of said screw to extend therethrough; 
 said first compressor stage having a drive connected to operate said screw; 
 said drive being a variable speed drive; 
 sensors mounted to monitor operation of said screw and said second compressor stage; 
 a controller operable to receive signals from said sensors and to control operation of said screw; and 
 operating said controller continuously to vary operating speed of said drive in response to operation of said second compressor stage; 
 said second compressor stage has a duty cycle, said duty cycle including a first portion and a second portion; 
 during said first portion of said second compressor stage duty cycle said controller is programmed to urge said screw to operate in a first speed regime, and 
 in said second portion of said second compressor stage duty cycle said controller is programmed to operate said screw in a second, different speed regime; and 
 said first portion of said duty cycle is a piston advancing portion and said second portion of said duty cycle is a piston retracting portion, and 
 said process including at least one of
 a) reducing speed of said screw during said piston retracting portion of said duty cycle, and 
 b) increasing speed of said screw during said piston advancing portion of said duty cycle. 
 
 
     
     
       10. The process of  claim 9 , the screw having a volute having wider pitch spacing at one end than the other, said process including progressively compressing said feedstock material with said volute as said screw turns. 
     
     
       11. The process of  claim 9  wherein said second compressor stage includes a piston, said piston being axially reciprocable relative to said screw, and said process includes adjusting operation of said screw while said piston is in operation. 
     
     
       12. The process of  claim 9  wherein said controller is programmed to monitor electrical motor current to said drive, and said process includes maintaining said electrical motor current below a maximum value during operation of said apparatus. 
     
     
       13. The process of  claim 9  wherein said process includes delivering feedstock from either of said compressor stages to a de-watering section and draining liquid from said feedstock at said de-watering section. 
     
     
       14. The process of  claim 9  wherein said process includes providing a pre-programmed schedule of operation for said second compression stage and conforming operation of said screw to said schedule.